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Identified virulence factors of Aeromonas: Toxin


Aerolysin  

Related genes: aerA/act;
Keywords: Toxin; Pore-forming toxin; Beta-barrel pore-forming toxin;
Structure features:
PDB code: 1PRE.
Aerolysin is produced as an inactive precursor, proaerolysin, which contains a C-terminal peptide (CTP) required for folding into its soluble form.
Proteolysis in the loop that connects the CTP to the main body allows aerolysin to oligomerize in a heptameric ring-like complex that inserts into the target membrane to form the pore. Proaerolysin is an L-shaped molecule.
Domain 1, involved in binding N-linked oligosaccharides.
Domain 2, involved in binding the glycan core of the glycosyl phosphatidylinositol (GPI)-anchored aerolysin receptors.
Domain 3, involved in oligomerization.
Domain 4, contains the CTP.
Figures:
Schematic model of the aerolysin-induced cellular response (From: Bucker R, et al., 2011. Aerolysin from Aeromonas hydrophila perturbs tight junction integrity and cell lesion repair in intestinal epithelial HT-29/B6 cells. J Infect Dis 204(8):1283-92.).


Functions:
Responsible for the hemolytic, cytotoxic, and enterotoxic activities.
Mechanism:
Secreted proaerolysin monomer binds to raft-associated GPI anchors on the apical membrane and oligomerizes into a heptameric β-barrel pore. Pore formation leads to active chloride secretion and potassium efflux. Aerolysin pore is permeable to calcium. [Ca2+] increasing leads to MLCK activation, which triggers myosin light-chain phosphorylation and actomyosin constriction. The subsequent tight junction (TJ) redistribution causes a leak flux. After cytoskeleton rearrangement, purse-string formation is inhibited, resulting in defective wound closure.
References:
Howard SP, et al., 1987. Nucleotide sequence of the gene for the hole-forming toxin aerolysin of Aeromonas hydrophila. J Bacteriol 169(6):2869-71.
Krause KH, et al., 1998. Aerolysin induces G-protein activation and Ca2+ release from intracellular stores in human granulocytes. J Biol Chem 273(29):18122-9.
Degiacomi MT, et al., 2013. Molecular assembly of the aerolysin pore reveals a swirling membrane-insertion mechanism. Nat Chem Biol 9(10):623-9.


RtxA (the repeat in toxin)  

Related genes: rtxA;
Keywords: Toxin; RTX toxin;
Characteristics:
A member of a protein family that is produced by a wide range of Gram-negative bacteria, such as Vibrio cholerae, V. vulnificus, V. anguillarum, Actinobacillus actinomycetemcomitans, Actinobacillus pleuropneumoniae,etc..
The rtx operon consists of six genes (rtxACHBDE) in which rtxA encodes an exotoxin, rtxC codes for an RtxA activator, rtxH encodes a conserved hypothetical protein and rtxBDE genes code for an ABC transporter.
Important characteristics of this toxin include:.
I) it requires post-translational modification, i.e., acylation to become biologically active.
II) has a COOH-terminal calcium-binding domain with tandem glycine/aspartic acid-rich repeats.
III) it has a high molecular mass of usually 100 to>400 kDa.
IV) it is delivered to the extracellular milieu through the T1SS.
Functions:
Inducing host cell rounding and apoptosis.
Mechanism:
Actin cross-linking domain of RtxA catalyzed the covalent cross-linking of the host cellular actin, thus disrupted the actin cytoskeleton of host cells.
References:
Suarez G, et al., 2012. Actin cross-linking domain of Aeromonas hydrophila repeat in toxin A (RtxA) induces host cell rounding and apoptosis. Gene 506(2):369-76.








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